1. Field of the Invention
The present invention relates generally to conductive, resin-based compositions, such as those which include thermally conductive fillers, which are useful as sealants, coatings, adhesives, and the like.
2. Brief Description of Related Technology
Advances in the electronic industry have made thermal management an increasingly important consideration, particularly with respect to packaging issues. For instance, heat build-up in electronic products leads to reduced reliability ("mean-time-to-failure"), slower performance, and reduced power-handling capabilities. In addition, continued interest in increasing the number of electronic components on, and reducing the size of, semiconductor chips, notwithstanding the desire generally to reduce power consumption thereof, also contributes to the importance of thermal management. Also, chip-on-board technology, where semiconductor chips are mounted directly to printed circuit boards, creates further demands on thermal management because of the more efficient use of surface area thereon. Thus, it is not surprising that packaging technology has been called one of the greatest single factors limiting the electronics industry. See M. M. Konarski and J. Heaton, "Electronic Packaging Design Advances Miniaturization", Circ. Assembly, 32-35 (August 1996).
Thermal management or heat dissipation techniques include generally convection or conduction mechanisms, where heat may be removed from electronic devices (such as operating silicon integrated circuits) by air (e.g., free flowing or forced) convection around the device, fluid (e.g., water or fluorocarbons) convection through radiators, or conduction through parts thereof which are in physical contact. A combination of such techniques is often used to maintain temperatures within design criteria.
Heat convection involves heat transfer across an interface which is proportional to (1) the amount of area exposed, (2) the temperature differential, and (3) the heat transfer coefficient, at the interface. Heat conduction, on the other hand, involves heat flow per unit area over a length which is proportional to the temperature gradient across that length. Thus, heat conduction (or thermal conductivity) is a steady-state property measuring the ability of a certain material to transfer heat therethrough. All else being equal, convection requires a larger surface area than conduction to allow the same amount of heat to dissipate. Of course, with continued size reduction of electronic packaging, surface area is reduced, thereby rendering convection less desirable.
A heat sink, constructed from a light weight thermally conductive material, such as aluminum alloy or graphite composite, is often used with electronic devices to facilitate heat dissipation therefrom. The heat sink should have sufficient mass to obtain a heat capacity which does not exceed a heat flow to the environment, which itself should be matched with heat flow from devices with which the heat sink is to be used.
Heat sinks have heretofore had varying measures of success, one reason for such variance is interfacial thermal resistance between the heat sink and the heat-generating electronic device. Generally, such resistance may be minimized by positioning at the interface junction between the electronic device and the heat sink a material having (1) high thermal conductivity, (2) intimate surface contact with the heat sink and electronic device, and (3) good durability, such as is measured by thermal cycling which detects failure or performance loss at the interface junction between the heat sink and the heat-generating device. Mechanical fasteners and thermally conductive greases, mica chips and ceramic insulators, pads and tapes, and adhesives have been used as such heat sinks or interface materials.
Mechanical fasteners are durable, but often provide high interfacial thermal resistance due to microscopic interfacial voids, which are present even in highly polished surfaces.
Surface contact with such fasteners may be improved using a thermal grease which penetrates such interfacial or surface voids, thereby effectively lowering interfacial thermal resistance. However, such greases generally tend to lack solvent resistance and often migrate over time out of the interface junction. Some users also consider thermal greases to be time-consuming and messy to apply, and difficult to cleanup. In addition, upon application of a thermal grease, solder processes should be avoided to minimize contamination. It is also advisable to avoid placing in cleaning baths thermal grease-containing components so as to minimize wash out of the thermal grease from the interface junction, the result of which would cause both a dry junction (and hence increased thermal resistance) and bath contamination.
Mica chips are inexpensive and have excellent dielectric strength; however, they are also brittle and easily damaged. In addition, mica itself has high thermal impedance, and as a result thermal greases are ordinarily also applied thereto. Ceramic insulators are costly and brittle, and thus easily damaged like mica chips.
Thermally conductive pads are laminated composite materials, which are often coated with pressure-sensitive adhesives to facilitate bonding and good thermal contact with the substrate surfaces between which they are positioned. See e.g., U.S. Pat. No. 4,574,879 (DeGree). Examples of such conductive pads include those commercially available from the W.R. Grace unit, Chomerics, Inc., Woburn, Massachusetts under the "CHO-THERM" trademark. The core of the pad generally is highly thermal conductive, while the coating itself is a compliant material having low thermal conductivity. Thus, thermal performance of conductive pads is often a function of mounting pressure and operating temperature, with the degree of surface penetration of the coating to the mating surfaces determining interfacial thermal resistance. Thermally conductive tapes perform in a like manner. See e.g., U.S. Pat. No. 5,510,174 (Litman).
Thermally conductive adhesives are curable (as contrasted to greases which are not intended to be curable) but like greases often contain thermally conductive fillers, commercially available examples of which include those supplied by Thermoset, Indianapolis, Ind. or Creative Materials Incorporated, Tyngsboro, Mass. These adhesives perform in a similar manner to greases, except that the adhesives, if formulated and applied properly and to appropriate surfaces, should not migrate from the interface junction.
Various thermally conductive adhesives are known for use in a number of applications, such as sealants, fuser roll coatings in electrostatic copying machines, bonding media, and the like. Resins employed in such compositions should themselves be thermally stable, examples of which include silicone, epoxy, phenolic, vinyl and acrylic materials. Silicones are particularly desirable resins because of, for instance, their high elasticity for stress relief, low moisture uptake, ionic purity, wide-range temperature performance, and excellent electrical properties, such as electrical insulating properties.
It is often desirable, however, to enhance the thermal conductivity of such adhesives, which of course depends on the conductivity of the resin itself. Improved thermal conductivity may often be attained by the addition of a conductive filler to the resin matrix. [See Handbook of Fillers for Plastics, 6.1, 255, H. S. Katz and J. V. Milewski, eds., Van Nostrand Reinhold Co., New York (1987); see also U.S. Pat. Nos. 4,147,669 (Shaheen)(gallium, aluminum, and gold, copper or silver in a resin); 4,544,696 (Streusand), 4,584,336 (Pate) and 4,588,768 (Streusand) (silicon nitride-containing organopolysiloxane with aluminum oxide or zinc oxide); 5,011,870 (Peterson) (aluminum nitride, and silicon metal and boron nitride in a polyorganosilicone resin matrix); and 5,352,731 (Nakano)(aluminum oxide-containing silicone rubber).]
U.S. Pat. No. 5,430,085 (Acevedo) describes a thermally and electrically conductive caulk including a resin, such as silicone, mixed with a filler which includes 80% by weight conductive particles with a particle size in the range of 300 to 325 microns, 10% by weight conductive particles with a particle size in the range of 75 to 80 microns, and 10% by weight conductive fibers having a length in the range of 0.020 to 0.025 inches.
U.S. Pat. No. 4,604,424 (Cole) describes thermally conductive silicone elastomers containing a polydiorganosiloxane, a curing agent, a platinum-containing hydrosilation catalyst, and zinc oxide and magnesium oxide fillers, the particle size of which fillers is such that substantially all of the filler particles pass through a 325 mesh screen, and the average particle size of which fillers is below 10 microns. The filler is composed of 50% to 90% zinc oxide, and 10% to 50% magnesium oxide, each by weight of the filler. Other fillers (up to 40% by weight) include aluminum oxide, ferric oxide and carbon black. The cured elastomers are said to resist erosion by abrasive materials to a greater extent than compositions containing aluminum oxide as the sole filler.
In U.S. Pat. No. 5,445,308 (Nelson), another method of improving thermal conductivity provides a connection between spaced surfaces by mixing a thermally conductive filler containing a liquid metal (e.g., gallium, gallium/indium, gallium/indium/tin and/or mercury) into an unhardened matrix material (e.g., thermoplasts, thermosets, UV-curable materials, epoxies and solvent-bearing materials) and thereafter hardening the matrix material.
An English-language abstract of Japanese Patent Document JP 07-292251 appears to relate to curable thermally conductive electrically insulating magnesium oxide-containing silicone compositions.
Chemical Abstracts CA 124:124432r (1996) refers to explosive compaction of aluminum nitride powders for use with silane elastomer precursors, which when polymerized are reported to have improved thermal conductivity of the so-formed polymer-ceramic composites.
Conductive particles are ordinarily manufactured for use though comminution of larger particles of a desired material into smaller particles by milling or grinding techniques and subsequent size segregation. Milling or grinding may be accomplished using a variety of mechanical mixing devices which provide intensive agitation to thoroughly mix together materials. In these devices, shear is produced, the degree of which depends on the speed at which contact is made between the agitation-producing components of the mixing device and the materials to be mixed as well as the mixer design. See Generally N. P. Cheremisinoff, Polymer Mixing and Extrusion Technology, Marcel Dekker, Inc., New York (1989).
Generally, it is noteworthy that percolation theory predicts that in the same volume a thermally conductive material having a larger particle size should demonstrate greater thermal conductivity than the same material having a smaller particle size. See e.g., 4 Encycl. Polym. Sci. & Eng'g, "Composites, Fabrication to Die Design", p. 343, FIG. 16, John Wiley & Sons, New York (1986). Accordingly, conventional wisdom would lead one to desire to maximize the particle size of conductive filler used in a filled resin.
Nevertheless, to date, it is not believed that such shear mixing has been used to comminute the particle size and geometric shape of a conductive filler with the intent of increasing the thermal conductivity of a resin throughout which it is dispersed.
Improvements in thermal management techniques are seen as necessary in order to foster commercially acceptable advances in the electronics industry. Thus, improved thermally conductive, resin-based compositions are a continuing challenge. There, therefore, is a need for conductive, resin-based compositions having superior conductivity characteristics without compromising the integrity of their mechanical properties or the mechanical properties of a cured reaction product.